Multi-layer structure having a thermal interface with low contact resistance between a microelectronic component package and a heat sink

ABSTRACT

A multi-layer solid structure and method for forming a thermal interface between a microelectronic component package and a heat sink so that the structure has a total thermal resistance of no greater than about 0.03° C.-in 2 /W at a pressure of less than 100 psi. The structure comprises at least two metallic layers each of high thermal conductivity with one of the two layers having phase change properties for establishing low thermal resistance at the interface junction between the microelectronic component package and the heat sink.

FIELD OF INVENTION

This is a Continuation-In-Part application of patent application Ser.No. 09/513,483 filed on Feb. 25, 2000, now U.S. Pat. No. 6,372,997, andthis invention relates to a thermal interface material having amulti-layer solid structure in which at least one thin outer surfacelayer has phase change properties and to a method for establishing athermal interface with low contact thermal resistance between amicroelectronic component package and a heat sink without theapplication of substantial clamping pressure.

BACKGROUND OF THE INVENTION

Microelectronic components, such as semiconductors, generate substantialheat which must be removed to maintain the component's junctiontemperature within safe operating limits. Exceeding these limits canchange the performance characteristics of the component and/or damagethe component. The heat removal process involves heat conduction throughan interface material from the microelectronic component to a heat sink.The selection of the interface material and the thermal resistance ofthe interface between the heat generating component (e.g. silicon icchip) and the heat sink controls the degree of heat transfer. As thedemand for more powerful microelectronics increase so does the need forimproved heat removal.

The thermal resistance between the microelectronic component package andthe heat sink is dependent not only upon the intrinsic thermalresistance of the interface material but also upon the contact interfacethermal resistance formed at the junction between the interface materialon each opposite side thereof and the microelectronic component and heatsink respectively. One known way to minimize contact thermal resistanceat each interface junction is to apply high pressure to mate theinterface material to the microelectronic package and heat sink.However, excessive pressure can create detrimental and undesirablestresses. Accordingly, the application of pressure is generally limitedso as not to exceed 100 psi and preferably below about 20 psi.

It is also known to use a thermal grease or paste as the thermalinterface material or to use a thin sheet composed of a filled polymer,metallic alloy or other material composition having phase changeproperties. A material having phase change properties is characterizedas having a viscosity responsive to temperature with the material beingsolid at room temperature and softening to a creamy or liquidconsistency as the temperature rises above room temperature.Accordingly, as the microelectronic component heats up the materialsoftens allowing it to flow to fill voids or microscopic irregularitieson the contact surface of the microelectronic component and/or heatsink. This allows the opposing surfaces between the microelectroniccomponent and heat sink to physically come closer together as the phasechange material melts thereby reducing the thermal resistance betweenthem.

Since the microelectronic package and heat sink do not generally havesmooth and planar surfaces a relatively wide and irregular gap may formbetween the surfaces of the microelectronic component and heat sink.This gap can vary in size from less than 2 mils up to 20 mils orgreater. Accordingly, the interface material must be of adequatethickness to fill the gap. The use of thermal grease, paste or phasechange materials cannot presently accommodate large variations in gapsizes. In general as the thickness of the interface material increasesso does its thermal resistance. It is now a preferred or targetedrequirement for a thermal interface material to have a total thermalresistance, inclusive of interfacial contact thermal resistance, in arange not exceeding about 0.03° C.-in²/W at an applied clamping pressureof less than 100 psi and preferably less than about 20 psi. Heretoforethermal interface materials did not exist which would satisfy thistargeted criteria.

SUMMARY OF THE INVENTION

A multi-layer solid structure and method has been discovered inaccordance with the present invention for forming a thermal interfacebetween a microelectronic component package and a heat sink possessinglow contact interfacial thermal resistance without requiring theapplication of high clamping pressure. Moreover, the multi-layerstructure of the present invention has thermal resistance propertieswhich do not vary widely over a gap size range of between 2-20 mils.

The multi-layer structure of the present invention is solid at roomtemperature and comprises a structure having at least two superimposedmetallic layers, each of high thermal conductivity with one of the twolayers having phase change properties for establishing low thermalresistance at the interface junction between a microelectronic componentpackage and a heat sink and with the thickness of the layer having phasechange properties being less than about 2 mils. High thermalconductivity for purposes of the present invention shall mean a thermalconductivity of above at least 10 W/m-K. The preferred class of highthermal conductivity metal carrier layers should be selected from thetransition elements of row 4 in the periodic table in addition tomagnesium and aluminum from row 3 and alloys thereof.

The preferred multi-layer structure of the present invention comprisesat least three layers having an intermediate solid core of a highthermal conductivity metal or metal alloy and a layer on each oppositeside thereof composed of a metallic material having phase changeproperties. A metallic material having phase change properties shallmean for purposes of the present invention a low melting metal or metalalloy composition having a melting temperature between 40° C. and 160°C. The preferred low melting metal alloys of the present inventionshould be selected from the group of elements consisting of indium,bismuth, tin, lead, cadmium, gallium, zinc, silver and combinationsthereof. An optimum low melting alloy composition of the presentinvention comprises at least between 19 wt %-70 wt % indium and 30 wt%-50 wt % bismuth with the remainder, if any, selected from the aboveidentified group of elements.

Another embodiment of the multi-layer structure of the present inventioncomprises a structure with at least one solid metallic layer of highthermal conductivity and a second layer having phase change propertiesfor establishing low thermal resistance at the interface junctionbetween a microelectronic component package and a heat sink, with saidsecond layer superimposed on a surface of said solid metallic layer suchthat a border of said solid metallic layer is exposed substantiallysurrounding said second layer. A preferred three layer structureincludes an intermediate solid metallic core with two opposing lowmelting alloy layers on opposite sides with each low melting alloy layersuperimposed on a given surface area on each opposite side of said solidmetallic core so as to form an exposed border of said solid coreextending substantially about said low melting alloy.

A preferred method of the present invention for forming a thermalinterface material comprises the steps of: forming a sheet of a highthermal conductivity material of predetermined geometry and thickness,treating at least one of said surfaces to form a treated surface adaptedto adhere to a low melting alloy and laminating a layer of a low meltingalloy upon said treated surface with the low melting alloy having athickness of no greater than about 2 mils. The preferred method oftreating the surfaces of the high thermal conductivity material topromote adhesion to a low melting alloy layer includes the step offorming dendrites on said high conductivity material which promotesadherence to the low melting alloy during lamination. Another preferredmethod of the present invention for forming a thermal interface materialcomprises the steps of: forming a sheet of high thermal conductivitymaterial of predetermined geometry and thickness with the sheet havingtwo opposite surfaces, treating at least one of said opposite surfaceswith an organic acid flux adapted to form a treated surface to which alow melting alloy will adhere when coated thereupon, and submersing saidsheet into a molten composition of a low melting alloy to form a thincoating of said low melting alloy on said treated surface with said thincoating having a thickness between 0.1 mil and 3 mils.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will become apparent from thefollowing detailed description of the invention when read in conjunctionwith the accompanying drawings of which:

FIG. 1 is a cross sectional view of a solid two layer thermal interfacematerial in accordance with the present invention with one layer havingphase change properties;

FIG. 2 is a cross sectional view of a solid three layer thermalinterface material in accordance with the present invention having twoopposing layers with phase change properties on opposite sides of ametallic core;

FIG. 3 is a cross sectional view of an alternate embodiment of the twolayer solid structure of the present invention;

FIG. 4 is a top view of embodiment of FIG. 3;

FIG. 5 is a cross sectional view of an alternate embodiment of the threelayer structure of the present invention;

FIG. 6 is a graph showing the correlation between thermal resistance ofthe thermal interface multi-layer solid structure of the presentinvention and thickness;

FIG. 7(a) is a schematic plan view of another embodiment showing themicroelectronics package and/or heat sink with the thermal interfacemulti-layer solid structure extending therefrom; and

FIG. 7(b) is a cross section of FIG. 7(a) taken along the lines 7—7.

DETAILED DESCRIPTION OF THE DRAWINGS

The thermal interface multi-layer structure of the present invention 10is solid at room temperature and comprises at least two metallic layers.The preferred arrangement of the two layer metallic structure of thepresent invention is shown in cross section in FIG. 1 and consists of asolid metal or metal alloy sheet 12 of high thermal conductivity,designated a carrier layer, and a superimposed low melting alloy sheet13 possessing phase change properties. The preferred three layerarrangement of the present invention is shown in cross section in FIG. 2consisting of an intermediate carrier layer 14 equivalent in compositionto the carrier layer 12 of FIG. 1 and two opposing layers 15 of a lowmelting alloy equivalent in composition to the low melting alloy layer13. In the embodiment of FIGS. 1 and 2 each low melting alloy layer 13or 15 is laminated over the entire planar surface of the high thermalconductivity layered sheets 12 and 14 respectively. In an alternateembodiment of the present invention as shown in FIGS. 3-5 a low meltingalloy layer 16, which may or may not be equivalent in composition to thelow melting alloy layers 13 and 15 of FIGS. 1 and 2, is laminated over asheet of a metallic high thermal conductivity material 18, equivalent incomposition to the layers of high thermal conductivity 12 and 14 ofFIGS. 1 and 2, so as to cover only part of the planar surface of thesheet of high thermal conductivity material 18 thereby forming a border19 which exposes a given surface area of the high thermal conductivitymaterial layer 18. This can be accomplished by masking an area on thehigh thermal conductivity material layer 18 before the low melting layeris coated thereon. Alternatively, a low melting alloy metal foil ofdesired geometry can be laminated to a larger size foil sheet of a highconductivity material to form the border 19. It is preferred that theborder 19 fully surround the pattern or footprint formed by the coatingof low melting alloy material 16 although the geometry of the border 19and the geometry of the coating of low melting alloy 16 are notessential to the present invention. Accordingly, the border 19 althoughshown in a rectangular geometry may be circular or of irregulargeometry.

In practical applications the multilayer structure 10 is placed betweena heat source (not shown) representing, for example, a microelectronicpackage having one or more integrated circuit chips and a heat sink (notshown) and may be compressed at any pressure up to 500 psi butpreferably at a pressure below 100 psi to form a thermal interface.Under heat and temperature generated by the microelectronic heat sourcethe low melting metal alloy melts and flows to fill up any voids orsurface irregularities existing on the interface surfaces of the heatsource and heat sink respectively. The alternative embodiments of FIGS.3-5 allow for the spread of the low melting alloy 16 over the exposedsurface area of the border 19 thereby preventing the escape of excessmolten metal alloy from the interface junction. In fact the surface areaof the border 19 to be formed can be calculated in advance for a givenamount of low melting alloy 16 so that essentially no excess metal willbe available to squeeze or drip out from the interface junction. In thearrangement of FIGS. 1 and 2 the low melting alloy layers must be verythin and preferably of less than 2 mils in thickness to minimize theamount of excess metal which may otherwise squeeze or drip out form theinterface junction.

The effectiveness of a thermal interface material is measured in termsof its overall or total thermal resistance. The units of thermalresistance are in ° C.in²/W. It has been found in accordance with thepresent invention that the low melting alloy layers can be of minimalthickness with the center or core material of the multi-layer structurevaried in thickness to accommodate different size gaps and with thethermal resistance of the multi-layer structure maintained below about0.03° C.-in²/W at a clamping pressure of less than about 100 psiindependent of gap size. To satisfy current microelectronic needs, asexplained earlier, the total thermal resistance for the thermalinterface material inclusive of its interfacial contact thermalresistance should not exceed about 0.03° C.-in²/W at a compression orclamping pressure of less than about 100 psi. Higher thermal resistanceequates to poorer performance and is unacceptable.

The following table A lists the thermal resistance of commerciallyavailable aluminum and copper foil at a thickness of 2 mils conductedunder modified ASTM D5470 standard at 45 watts and 20 psi.

TABLE A Thickness Thermal Resistance Foil (mils) (° C.in²/W) Aluminum2.0 0.129 Copper 1.8 0.159

Table B, as shown below, lists the thermal resistance of severaldifferent low melting alloy foil compositions under the same ASTMstandard as that of Table A at 45 watts and 20 psi. The composition oflow melting alloy 162 is: 66.3 wt % In and 33.7% wt Bi. The compositionof low melting alloy 19 is: 51 wt % In, 32.5 wt % Bi and 16.5 wt % Snwhereas the composition of low melting alloy 117 is: 44.7 wt % Bi, 22.8wt % PB, 19.1 wt % In, 8.3 wt % Sn and 5.3 wt % Cd.

TABLE B Foil Thickness Thermal Resistance Low Melting Alloy (mils) (°C.in²/W)  19 2.0 0.010 162 2.0 0.009 117 2.0

A thin film of a low melting metallic alloy having phase changeproperties may be laminated to a solid carrier of a metallic highthermal conductivity material to form the multi-layer structure of thepresent invention. Any thermal conductivity material may be used havinga thermal conductivity of above at least 10 W/m-K inclusive of any ofthe transition elements of row 4 in the periodic table in addition tomagnesium and aluminum of row 3 and their alloys. However, a foil sheetof either aluminum or copper as the carrier layer is preferred.

In accordance with the present invention to laminate or coat a thinlayer of a low melting metallic alloy of less than about 2 mils inthickness to a foil sheet of copper or alumimum requires treating thesurface or surfaces of the sheet of copper or alumimum to be coated topromote the adhesion of the low melting metallic alloy. Otherwise thethin surface layers of the low melting alloy readily delaminate i.e.,physically separate from each other. In fact a thin layer of a lowmelting metallic alloy cannot be swaged to a sheet of untreated copperor aluminum foil even at very high pressures without causingdelamination. However, if the surfaces of the copper or aluminum foil tobe laminated are treated in accordance with the present invention a thinlayer of a low melting metallic alloy of less than 2 mils in thicknesscan be readily laminated or coated upon the copper or aluminum foil toform the integrated solid multi-layer structure of the presentinvention. The treatment involves forming dendrites on the surface ofthe metal foil to be laminated or by application of an organic acid fluxto the surface of the metal foil to be coated. The dendrites formprotrusions that form an interlocking structure with the low meltingalloy during lamination. Treating a metal surface to form dendrites onthe surface or treating a metal surface with an organic acid flux areknown techniques but not for the purpose of assembling a multi-layerthermal interface structure as taught in the present invention.

For example it is known that a copper surface can be treated to form acontrolled surface topography of dendrites by electrochemical etchingwith an oxide or zinc or brass for forming dendritic sites. Fluxing ametal surface by application of an organic acid flux is also well knownto improve the solderability of the surface. An organic acid flux isknown to contain an organic acid preferably glutamic acid hydrochlorideand polyethlyene glycol or a polyglycol surfactant and may include ahalide containing salt and amines as well as glycerine.

The following are examples of the multi-layer thermal interfacestructure of the present invention.

EXAMPLE 1

One and two ounce copper foil treated on both sides to form dendriteswas used to make a three layered sandwich structure formed of alloy 162-copper-alloy 162. The three layers were swaged together. Samples weredie cut, with no delamination, and with thermal resistance measured asshown below resulting in very low thermal resistance with essentially nodifference between one ounce and two ounce foil carriers. The thicknessof each 162 alloy layer was 2 mils.

TABLE C Sample/ Carrier thickness Pressure Thermal Resistance type(mils) (psi) (° C.in²/W) 162/1 ozCu/162 1.4 20 0.011 162/1 ozCu/162 2.820 0.012

EXAMPLE 11

A multi-layer composite with a carrier layer material of copper and alayer of a low melting indium alloy on opposite sides thereof was testedwith the thickness of the carrier layer varied as shown below in TableD:

TABLE D Core/ Core/Thickness pressure Thermal Resistance material alloyfoil (mils) (psi) (° C.in²/W) 1 oz Copper None 1.4 20 0.283 1 oz Copper162 1.4 20 0.010 2 oz Copper 162 2.8 20 0.011 3 oz Copper 162 4.2 200.011 4 oz Copper 162 5.6 20 0.012 6 oz Copper 162 8.4 20 0.019

The tests in the above table D were conducted to show that differencesin thickness of the core material do not materially change the thermalresistance of the sandwich multi-layer structure. The thermal resistanceis due more to the interface resistance between component and compositethan to the inherent thermal resistance due to thickness changes in thecore material. This is because copper has a thermal conductivity ofabout 300 W/mK, and therefore does not contribute significantly to thethermal resistance.

EXAMPLE 111

A similar test was conducted using aluminum as the core material withthe results shown below in Table E:

TABLE E Carrier/ Carrier/Thickness Pressure Thermal Resistance materialAlloy (mils) (psi) (° C.in²/W) Aluminum 162 2.00 20 0.01  Aluminum 16286.5 20 0.029 Aluminum 162 250 20 0.063

The tests in the above table E clearly show that differences inthickness of the aluminum do not materially change the thermalresistance of the sandwich multi-layer structure even for drasticchanges in aluminum thickness.

If the thermal resistance of the three composite samples are plottedagainst thickness (x axis) as shown in FIG. 6 the thermal conductivitycan be calculated. The thermal conductivity is the inverse of the slopeof the graph and may be calculated by the following equation:

Thermal conductivity=1/slope

Thermal conductivity=1/0.2128° C.in/W·39.4 in/m=185 W/mK

The measured thermal conductivity is reasonable for aluminum.

EXAMPLE 1V

Experiments were also run on a multi-layer structure as shown below inTable F to show that the surface finish of the platens would have littleeffect on the thermal performance of the sandwich structure. The exactsurface finish of the roughened platens is not known but the gap with nointerface material, referred to as “dry gap” was measured for theroughened surfaces to compare with the normal platens polished to asurface finish of 0.4 micrometers.

TABLE F Carrier Thermal Thickness Pressure Resistance Sample Platens(mils) (psi) (° C.in²/W) Dry gap polished 0 20 0.051 Dry gap roughened 020 0.231 162/2 ozCu/162 roughened 2.8 20 0.021 162/2 ozCu/162 polished2.8 20 0.012

The above table F shows that the sandwich thermal resistance is notaffected greatly by differences in surface finish and tolerance issues.

EXAMPLE V

An alternative method has been discovered in accordance with the presentinvention for forming a thin coating of a low melting metallic alloymaterial with a thickness of below 1 mil on a sheet of high thermalconductivity metal following the treatment of the surface(s) to becoated to promote adhesion as explained heretofore by coating thetreated surfaces with the low melting metal alloy composition by meansof any conventional coating technique. An example of this alternativemethod follows involving submerging the sheet of high thermalconductivity metal into a molten bath of low melting metallic alloy.

3.5 pounds of a low melting alloy comprised of indium, bismuth and tinwas placed into an 8 inch square pyrex dish and brought to a temperatureof 95° C. in a forced air oven. The sample was prepared from 2 ouncerolled annealed double treated copper foil (i.e., treated on both sidesto form a dendritic surface) and also treated on both sides thereof withan organic acid flux preferably including glutamic acid hydrochlorideand polyethlyene glycol. The preferred organic acid flux as aboveidentified is commercially available from the Superior FluxManufacturing Company. The oven door was opened and the foil submergedinto a molten bath of the low melting molten alloy for 30 seconds. Thefoil was pulled out of the molten alloy and excess alloy allowed to flowback into the bath. After the molten alloy was allowed to resolidify, ahot air gun was used to reflow the alloy and blow off excess until auniform coating of 0.0005 inches was formed on both sides. The thermalresistance was measured to be 0.01° C.in²/W.

The above procedure may also be used to form a configuration as shown inFIGS. 3-5 by masking a sheet of foil of the high thermal conductivitymetal using, for example, Kapton Tape, to mask the border on the treatedsurface(s) of the sheet of metal before submerging the sheet into thelow melting molten metal alloy composition. The foil is then taken outin the same way and the tape removed leaving an uncoated border.Alternatively, an organic acid flux can be applied to specific locationson the foil surface prior to dipping causing the low melting alloy toadhere only to the surface areas treated with the organic acid flux.

In another embodiment of the present invention as shown in FIGS. 7a and7 b the free standing thermal interface multi-layer structure 10 of thepresent invention is constructed of a size so that its surface area islarger than the surface area 1 of the microelectronics package and/orheat sink. This is illustrated in FIG. 7a wherein the free standingthermal interface multi-layer structure 10 is of a size so that itssurface area extends from the microelectronics package and/or heat sink1 to form an exposed border area 2.

The exposed border area 2 as is shown in FIG. 7a is deliberatelyexaggerated for purposes of illustration and need only be in a range ofbetween 1 to 10 mm but preferably between 2-5 mm. The purpose of theexposed border 2 is to provide an area surrounding the microelectronicspackage and/or heat sink 1 which will act as a wetting surface to entrapthe low melting alloy having phase change properties which is squeezedout from the multi-layer structure 10 once the phase change materialsoftens or melts in response to an elevation in temperature at thethermal interface. The melted or softened phase change alloy materialwill exit from between the microelectronics package and the heat sinkand make contact with the exposed border area 2 causing it to solidifyin the form of a bead 5 on the border area 2. The bead 5 formed from thesolidification of the low melting phase change alloy material is shownin FIG. 7b as a beaded mass surrounding the microelectronics packageand/or heat sink. If no border area existed, the low melting phasechange alloy material upon being squeezed out from between themicroelectronics package and heat sink would freely migrate from themicroelectronics package to other electronic parts resulting in possibleelectrical shorts which can be catastrophic to the viability of themicroelectronic package.

What is claimed is:
 1. A free standing multi-layer solid metallicthermal interface structure for placement at the interface between amicroelectronic component package and a heat sink so as to provide atotal thermal resistance of no greater than about 0.03° C.-in²/Wcomprising at least two solid metallic layers of high thermalconductivity superimposed upon one another to form a sandwich in crosssection with one of the two solid metallic layers having phase changeproperties and a thickness of less than about 2 mils and wherein thesurface area of the interface structure is larger than the surface areaof the microelectronic component package and/or heat sink such that theperimeter of said free standing thermal interface structure extends fromthe microelectronic component package and/or heat sink to form anexposed border area surrounding the microelectronic component packageand/or heat sink.
 2. A multi-layer solid metallic thermal interfacestructure as defined in claim 1 comprising at least three highconductivity layers having an intermediate solid core composed of a highthermal conductivity metal or metal alloy and a solid layer on eachopposite side thereof composed of a metallic material having phasechange properties with each layer on each opposite side of said solidcore having a thickness of less than about 2 mils.
 3. A free standingmulti-layer solid metallic thermal interface structure as defined inclaim 2 wherein said exposed border area extends a distance of between 1and 10 mm from the perimeter of the microelectronics component packageand/or heat sink.
 4. A free standing multi-layer solid metallic thermalinterface structure as defined in claim 1 wherein said exposed borderextends a distance of between 2-5 mm from the perimeter of themicroelectronics package and/or heat sink.
 5. A multi-layer solidmetallic thermal interface structure as defined in claim 3 wherein thecomposition of said solid core is selected from the transition elementsof row 4 in the periodic table in addition to magnesium and aluminumfrom row 3 of the periodic table and alloys thereof.